The FCC, or fluidized catalytic cracking process, is a mature process. It is used to convert relatively heavy, usually distillable, feed to more valuable lighter products. There is an increasing need in modern refineries to convert more of the "bottom of the barrel", e.g., resids or residual oil fractions to more valuable lighter products.
In the past these heavy streams were subjected to various thermal processes. Unfortunately, thermal processing alone has not proved to be a complete answer to the problem, as the products of thermal cracking are themselves relatively low valued products, such as heavy fuel oil from visbreaking or coker naphtha or coker gas oil from coking.
Residual oils have a large percentage of refractory components such as polycyclic aromatics which are difficult to crack. Resids also contain large amounts of metals which rapidly deactivate conventional catalyst. Some attempts at catalytic processing of these stocks have been made, e.g., adding relatively small amounts of residual oil to conventional FCC feed. FCC units can tolerate modest amounts of resids in the feed, e.g., 5-10 wt. percent but the heavy feeds (because of their high Conradson carbon content) increase the burning load on the regenerator and poison the catalyst, with nickel and vanadium. Limiting the amount of resid in the FCC feed has been the method of choice in controlling regeneration temperature, although consideration has been given to adding catalyst coolers. The nickel and vanadium contamination problem can be overcome to some extent by practicing metals passivation, e.g., addition of antimony to the unit to passivate the metals added with the feed. Metals passivation has allowed FCC units to continue operating with catalyst containing relatively high amounts of nickel and vanadium, but not been a complete solution. Nickel is passivated, but vanadium remains as a poison. The vanadium seems to attack the zeolite structure of modern FCC catalyst, resulting in rapid loss of catalyst activity. The exact cause of vanadium poisoning is not completely understood, but it is believed that pentavalent vanadium compounds are formed in the highly oxidizing atmosphere of conventional FCC regenerators. These compounds, particularly vanadic acid, rapidly attack the zeolite. The problem of vanadium contamination in FCC catalyst is discussed in S. G. Jarzas, Applied Catalysis, 2, 207 (1982).
Although additive materials which are selective for nickel, vanadium, and other metals in the feed can be added to compensate for higher metals feed, there are still some problems associated with this approach.
High surface area getter materials, such as alumina, have a much greater affinity for e.g., vanadium, than does the conventional cracking catalyst. There is still a competition between the alumina additive and the conventional FCC catalyst for the metals content of the hydrocarbon oil. It is very difficult to have high capture of metals on the additive, because the cracking catalyst itself contains silica/alumina as a matrix which is also an efficient metals getter. There is much catalyst, and only minor amounts of additive, so much of the metal in the feed ends up on the catalyst.
The problems are compounded by the changes that occur in the regenerator when heavy, metals laden crudes are processed. Usually these heavy materials have associated with them an abundance of coke precursors, e.g., high Conradson carbon levels. This leads to increased coke deposition on the catalyst, and increased heat generation (and higher temperatures) in the regenerator. The high temperatures and steam (from stripping steam and water of combustion of the hydrogen in hydrocarbonaceous coke) create a severe environment for long term stability of the zeolites. The vanadium levels also promote attack of the zeolite structure.
Processes trying to operate with heavy, metals laden feed, generally tried to overcome the problems associated with high temperature in the regenerator and high metals levels by adopting a more aggressive catalyst withdrawal/makeup procedure to keep the metals level on the catalyst at an acceptable level. When resort has been made to a getter material, this has allowed some reduction in catalyst withdrawal/makeup rates, but quite a lot of getter material must be added to efficiently act as a sink for vanadium, because of the aforementioned competition between the getter material and the conventional catalytic cracking catalyst.
We have now discovered a catalyst which can be used in these severe conditions and which tends to avoid the vanadium problem. We eliminate from the catalyst surface all materials which would act as efficient vanadium sinks. In effect, we have created "Teflon catalyst" to which no vanadium will stick.